1. Field
The present invention relates to technology for non-volatile storage.
2. Description of the Related Art
Semiconductor memory has become more popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrical Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories.
Both EEPROM and flash memory utilize a floating gate that is positioned above and insulated from a channel region in a semiconductor substrate. The floating gate is positioned between the source and drain regions. A control gate is provided over and insulated from the floating gate. The threshold voltage of the transistor is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate. Thus, a memory cell (which can include one or more transistors) can be programmed and/or erased by changing the level of charge on a floating gate in order to change the threshold voltage.
Each memory cell can store data (analog or digital). When storing one bit of digital data (referred to as a binary memory cell), possible threshold voltages of the memory cell are divided into two ranges which are assigned logical data “1” and “0.” In one example of a NAND type flash memory, the threshold voltage is negative after the memory cell is erased, and defined as logic “1.” After programming, the threshold voltage is positive and defined as logic “0.” When the threshold voltage is negative and a read is attempted by applying 0 volts to the control gate, the memory cell will turn on to indicate logic one is being stored. When the threshold voltage is positive and a read operation is attempted by applying 0 volts to the control gate, the memory cell will not turn on, which indicates that logic zero is stored.
A memory cell can also store multiple levels of information (referred to as a multi-state memory cell). In the case of storing multiple levels of data, the range of possible threshold voltages is divided into the number of levels of data. For example, if four levels of information is stored, there will be four threshold voltage ranges assigned to the data values “11”, “10”, “01”, and “00.” In one example of a NAND type memory, the threshold voltage after an erase operation is negative and defined as “11.” Positive threshold voltages are used for the states of “10”, “01”, and “00.” If eight levels of information (or states) are stored in each memory cell (e.g. for three bits of data), there will be eight threshold voltage ranges assigned to the data values “000”, “001”, “010”, “011” “100”, “101”, “110” and “111.” The specific relationship between the data programmed into the memory cell and the threshold voltage levels of the memory cell depends upon the data encoding scheme adopted for the memory cells. For example, U.S. Pat. No. 6,222,762 and U.S. Patent Application Publication No. 2004/0255090, both of which are incorporated herein by reference in their entirety, describe various data encoding schemes for multi-state flash memory cells. In one embodiment, data values are assigned to the threshold voltage ranges using a Gray code assignment so that if the threshold voltage of a floating gate erroneously shifts to its neighboring physical state, only one bit will be affected. In some embodiments, the data encoding scheme can be changed for different word lines, the data encoding scheme can be changed over time, or the data bits for random word lines may be inverted to reduce data pattern sensitivity and even wear on the memory cells. Different encoding schemes can be used.
When programming an EEPROM or flash memory device, such as a NAND flash memory device, typically a program voltage is applied to the control gate and the bit line is grounded. Electrons from the channel are injected into the floating gate. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the threshold voltage of the memory cell is raised so that the memory cell is in a programmed state. More information about programming can be found in U.S. Pat. No. 6,859,397, titled “Source Side Self Boosting Technique For Non-Volatile Memory,” and in U.S. Patent Application Publication 2005/0024939, titled “Detecting Over Programmed Memory,” both of which are incorporated herein by reference in their entirety. In many devices, the program voltage applied to the control gate during a program operation is applied as a series of pulses in which the magnitude of the pulses is increased by a predetermined step size for each successive pulse.
Many non-volatile storage systems include an array of memory cells arranged in columns and rows. Control lines (e.g., word lines, bit lines, or other types of control lines) connect to the various rows and columns. In one example, word lines are used to access rows of memory cells while bit lines are used to access columns of memory cell. In this arrangement, the series of pulses of the program voltage are applied to a selected word line that is connected to a set of selected memory cells. Each of the selected memory cells receiving the pulses potentially has its threshold voltage raised in response thereto. As the memory cells reach their target threshold voltage, they are locked out from further programming. It has been observed that as memory cells become locked out, they interfere with the expected programming rate of their neighbor memory cells. This effect can cause the neighbor memory cells to overshoot their intended target threshold voltage and, therefore, become over-programmed. In some cases, an over-programmed memory cell will cause an error when being read.